1. Field of the Invention
The present invention relates to a tire rubber composition (i.e., a rubber composition for a tire), in particular, a tire rubber composition that can give compatibility between tire-abrasion resistance and ice braking-performance. The tire rubber composition according to the present invention is useful as a rubber composition for a studless tire.
2. Description of Related Art
In automobiles that run on icy road surfaces, studless tires, the tread portions of which are basically made of block-like material, are generally used. For a tread rubber which constitutes the tread portions, a method of incorporating a hard material, such as hollow particles, glass fiber or plant granules, thereinto is adopted in many cases in order to improve the grounding performance thereof onto icy road surfaces. For example, Patent document 1 describes a tire rubber composition into which plant granules subjected to surface treatment are incorporated, Patent document 2 describes a tire rubber composition into which a fraipontite-silica composite is incorporated, and further Patent document 3 describes a tire rubber composition into which a porous carbide of a plant is incorporated.
However, even when the above-mentioned hard material is incorporated thereinto, it is difficult to improve the braking performance on icy road surfaces (hereinafter referred to as the “ice braking-performance”) while the tire-abrasion resistance is kept. Thus, there remains a room for a further improvement.
Patent document 4 describes a method of incorporating a non-woody fiber further into a silica-containing tread rubber composition for a tire, thereby decreasing the electrical resistance of the tread rubber. However, this patent document neither describes nor suggests a method of making the tire-abrasion resistance compatible with the ice braking-performance.
Patent document 5 describes a tire rubber composition into which crosslinked rubber particles and plant granules are incorporated. By the combination of the crosslinked rubber particles with the plant granules, a pneumatic tire produced from this tire rubber composition is largely improved in performance on ice and snow. However, a further improvement in the performance is requested in the market. In the actual circumstances, pneumatic tires having properties over the advantageous effects which these compositions produce are requested.    Patent document 1: JP-A-10-7841    Patent document 2: JP-A-2001-123017    Patent document 3: JP-A-2005-162865    Patent document 4: JP-A-2002-69243    Patent document 5: JP-A-2008-24792